To detect tau protein deposits in the living brain, [18F]THK-5105 and [18F]THK-5117 have been developed as tau PET probes. In vitro binding assay and autoradiography demonstrated the high binding affinity and selectivity of these probes to tau pathology in Alzheimer's disease (AD) brains. First-in-man PET studies have been performed to assess the clinical usefulness of these probes. AD patients showed THK-5105 and THK-5117 retention in the lateral and medial temporal cortices that are known to contain high density of tau deposits. Regional distribution of these tracers differed considerably from that of PiB. In addition, the retention of these tracers was correlated with clinical severity and brain atrophy in AD patients. From these findings, [18F]THK-5105 and [18F]THK-5117 are considered to be useful for the non-invasive assessment of tau pathology.